<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: red;
      transition-property: background-color;
      transition-duration: 1s;
      transition-timing-function: linear;
    }
    div:hover {
      background-color: green;
    }
  </style>
</head>
<body>
  <div>鼠标放上来</div>
</body>
</html>